Ground proximity warning systems that provide warnings of potential impact with the ground under controlled flight conditions have been developed over the past fifteen years. Examples of such systems are disclosed in U.S. Pat. Nos. 3,946,751; 3,947,810; 4,060,793; 4,319,218 and 4,433,323. One of the objects of the ground proximity warning systems illustrated in the above patents is to utilize sensors that are normally present in commercial aircraft, such as the radio altimeter, barometric altimeter and glide slope receiver to provide the aircrew with timely warnings of an impending but inadvertent contact with the ground. These systems have generally proved to be highly effective in preventing controlled flight into terrain type accidents.
However, there are flight situations where the performance of the aircraft itself becomes degraded and in certain of these situations existing ground proximity warning systems may not provide as timely a warning as might be desired. Reasons for degraded aircraft performance are many and varied and as such include: wind shear, etc.; improper configuration including gear down, partial spoilers, flaps, etc.; degraded lift from rain, ice, excess weight, improper flap settings, etc.; insufficient engine thrust; and instrument errors leading to inappropriate changes in thrust, attitude or airspeed. When reviewed with respect to past aircraft accidents involving degraded performance neither existing ground proximity warning Mode 1 which is the excessive descent rate warning mode described in U.S. Pat. No. 4,060,793 nor Mode 3 which is the negative climb after take-off warning mode described in U.S. Pat. No. 4,319,218 would always provide as much warning as might be desired For example, in certain wind shear situations the warning generated by existing Modes 1 and 3 may not be timely enough to be useful.
In addition to giving timely alerts it is also highly desirable to give the aircrew an indication as to what should be done to recover from a dangerous situation especially under unusual circumstances such as wind shear or misleading instrument readings. For instance, there have been situations where an aircraft has struck the ground which could have been avoided if the aircrew had appreciated that the aircraft had additional performance immediately available in terms of airspeed that could have been converted to altitude or that additional thrust could have been applied.
With respect to degraded performance due to wind shear, there have been a number of proposed systems, as described, for example, in U.S. Pat. Nos. 4,043,194; 4,079,905; 4,229,725; 4,281,383; 4,342,912 and 4,336,606, for alerting an aircrew to a wind shear condition. However, such systems are often difficult to implement or require additional sensors or do not provide usable information in a timely manner.
In one approach described in U.S. Pat. No. 4,189,777, airspeed rate is used to detect a wind shear condition and in response thereto a ground proximity warning system Mode 1 warning curve is modified to increase warning time. Another approach relating to wind shear conditions is described in U.S. Pat. No. 4,347,572 in which angle of attack, stick shaker value, vertical speed, airpseed, flap position, and thrust are used to provide climb out guidance on a pilot flight director display in a wind shear situation.
None of the systems described above provide enhanced ground proximity warning or guidance for a comprehensive set of degraded aircraft performance situations.